Since Arceus has Base 120 Stats across the board it has Decent bulk to set up a Swords Dance enabling Arceus to Sweep. Silk Scarf Boosts the power of Arceus' Normal type moves and not at the cost of recoil damage unlike Life Orb. Extreme Speed is STAB and it has Priority. Swords Dance raises its Attack to two stages enabling Arceus to Sweep like I said. Shadow Claw hits the ghost types that are immune to Extreme Speed like Giratina along with its boosted critical hit ratio. Brick Break or Earthquake is Coverage against Rock and Steel. Types that wall Arceus' STAB. Brick Break shatters the likes of Light Screen and Reflect or Earthquake has more base power.

Well, I'd say punishment is quite useless in your set since you know it deals damage according to the opponents' increased stats and you have no way to tell whether the opponent you face has stat increasing moves or not. And normally it has a base power of only 60 not making it the best of options for a moveset. So I think you should get rid of punishment.
However, beware of the fact that once you get rid of punishment you will have no way to hit Pokemon like Haunter or Rotom. I suggest you instead teach it swagger instead of extremespeed and get rid of seismic toss. Using swagger once on the opponent is risky but that results in the power of punishment rising upto 100 which is pretty good in my opinion.
Also, if you really wanna get extremespeed then get swords dance too and get a move to counter ghost types too. As you might have already heard of the sweeping Arceus, it can easily set up two swords dances and then really sweep teams with extremespeed.
So there you go, quite a number of possibilities for you to explore. Hope I helped! :)))